A gambler who lost $500,000 (€360,000) at a Las Vegas casino is suing the venue for his losses – because he claims he was allowed to play despite being visibly drunk.

US businessman Mark Johnston, 52, said he should not have to pay his gambling debts after he arrived at the Downtown Grand Las Vegas Hotel and Casino and was, he claims, plied with free alcoholic drinks while he played.

Mr Johnson alleges that staff served him at least 20 drinks during the 17 hours he spent gambling at the casino even though he was visibly intoxicated, according to the suit filed 18 February in Nevada state court for Clark County.

He said he started playing in the evening of 30 January and finally stopped the next afternoon.

Under Nevada law, casinos are not allowed to serve visibly drunk patrons.

After leaving the gaming tables, Mr Johnston went to his hotel room and woke up the next day with no memory of his time at the tables, the lawsuit states, which described his mental state while gambling as a "blackout period."

"Just picture a drunk walking the street and he's drunk, and someone pickpockets and takes his money from him. That's how I characterise it," Mr Johnston told CNN.

"My responsibility is, look, I had some drinks at the airport, on the plane. At some point, that's my responsibility," he added. "The unfortunate part about it for them is, they have a bigger responsibility than I do."

Mr Johnston, from Ventura, had been given credit in the amount of $250,000 (£149,000), and claims the amount was increased while he was gambling so he ultimately lost $500,000.

"Mr Johnston, an experienced gambler, was dropping chips on the floor, confusing chip colours and slurring his speech badly, and he was unable to read his cards or set his hands properly," the lawsuit states.

This description of his behaviour came from a staff member working at the casino during Mr Johnston’s time there, who has since stopped working at venue, according to Mr Johnston’s lawyer Sean Lyttle.

Mr Lyttle described his client’s case as “extraordinary”.

"Someone was blackout intoxicated where they couldn't read their cards, and yet a casino continued to serve them drinks and issue them more markers," he told The Associated Press.

The casino has so far declined to comment. The Nevada gaming control is reportedly investigating the matter.